DbContext has been disposed

Your context has been disposed somewhere else (not in the code you've shown), so basically when you access it from your Register action, it throws the exception.

Actually, you shouldn't use a static singleton to access to your context. Do instantiate a new DbContext instance for each request. See c# working with Entity Framework in a multi threaded server

In my case, my GetAll method was not calling ToList() method after where clause in lambda expression. After using ToList() my problem was solved.

Where(x => x.IsActive).ToList();

You are probably 'lazy-loading' a navigation property of User in your registration view. Make sure you include it by using the Include method on your DbSet before sending it to the view:

_db.Users.Include(u => u.PropertyToInclude);

Also, sharing DbContexts with a static property may have unexpected side effects.